Computer-readable optical media discs are used to store data in a digital form that can be readily retrieved from the disc by an appropriate disc reading device. These discs include CD-ROMS (Compact Disc Read Only Memory), rewritable CDs, and DVDs (Digital Versatile Discs). DVDs are functionally similar to CDs, except that a single sided DVD can store about 4.3 gigabytes of data in comparison to the 680 megabyte storage capacity of a CD. For simplicity, the following will generally discuss compact discs. The principles also apply to single and double sided DVDs as well.
CDDA (Compact Disc Digital Audio) discs were the first optical media discs to store digital data. They are commonly known as Compact Discs or CDs. They were originally developed to store audio content, and have become the de facto standard for such, replacing phonographic records. The widespread success of CDs led to the development of CD-ROMs, which have become a standard medium for storing computer-readable information. CD-ROMs are read by equipment called CD-ROM drives. Modern CD-ROM drives and associated computer hardware and software drivers enable today's computers to read data, audio, and video information from CDs.
Information content is stored on a CD in a binary format comprising a sequence of on and off bits. A CD is made from a polycarbonate disc that has a specially treated reflective surface that is encoded with a pattern of “pitted” and “non-pitted” areas. These pitted and non-pitted areas are formed along a continuous spiral that starts from near the inner edge of the disc, extending outward toward the outer edge of the disc, much like the single groove on a phonographic record. A tracking laser and photosensor head (read-head) are controllably situated above the spiral grove as the compact disc is rotated on a platter by a motorized device so as to read the binary pattern formed by the pitted and non-pitted areas. When the laser falls upon a pitted area the laser light is reflected in such a way that the amount of light detected at the photosensor is vastly reduced from the amount of light detected at the photosensor when the laser light reflects from a non pitted area. As the CD is rotated and the spiral of pits passes underneath the read-head, the photosensor and associated electronics convert this variation in detected light intensity into a stream of digital bits or logical ones and zeros. After further processing by error detection and correction circuitry, the stream of bits is converted into a stream of output data bytes. These data bytes can be PCM audio data, computer file system data, or other digital data. These data bytes can be read in chunks called blocks or sectors by a computer. The rate at which the data on a CD can be read is called the transfer rate.
When CD technology was originally developed, engineers had to determine the density limitations of the pitted pattern inscribed on the CD so as to meet playback accuracy and manufacturing needs based on the existing technology at the time. A density of about 150 megabytes per square inch was the result of the original CD standards produced in the early 1980's. The CD standards prescribe the dimensions and tolerances for the pit width, depth, and length, as well as the track pitch (how far the pit spiral advances towards the outside of the disc for each revolution of the disc). The first CD standard described CDDA specifications and became known as the Redbook, since the color of the book containing the standard was red. Succeeding standards regarding CDs are called Yellow Book, Orange Book, etc. The Redbook standard specifies a standard track pitch of 1.6 micrometers, pit width of 0.6 micrometers and the minimum pit length of 0.83 micrometers.
Since the original CD equipment was designed for the reproduction of audio content, it was the objective of the engineers to develop a means for accurately reproducing the audio content for real-time playback. The sample rate of CDDA digital audio is a constant 44.1 kHz for the duration of all recordings. Therefore, the playback method which minimizes the amount of storage required in the playback device, while still maximizing the storage capacity of the digital storage medium by maintaining a constant storage density across the disc, is one in which the disc is spun at a varying speed so as to maintain a constant bitrate independent of the location of the photosensor along the pit spiral. The original CD players were designed in this way. As the read head of the CD moves out along the pit spiral towards the outside of the disc, the player slows the motor spinning the disc. The player thus maintains a constant bitrate just sufficient to reproduce the digital audio samples at the 44.1 kHz rate at which they are being consumed by the DACs in the player. A CD audio player maintains a constant linear velocity of about 1.2 meters per second. In order to maintain this constant linear velocity a CD player spins the CD at about 400 RPM when the read-head is close to the inner edge disc, and slows the CD to about 200 RPM as the read-head reaches the outer edge of the disc.
The first generation CD-ROM drive design was based on the audio CD player design and thus CD-ROMs inherited this constant linear read-head velocity design. This type of CD-ROM drive is known as a Constant Linear Velocity or CLV CDROM drive. With a CLV CD-ROM drive the data transfer rate is the same regardless of where the data on the CD resides.
When a CD-ROM drive is used to retrieve digitally stored data from a CD-ROM there is no requirement that the data be transferred at the same rate as the original audio CD players. Rather, the objective is to transfer the data as fast as possible, while still maintaining accuracy. Advancement in lasers, photosensors, read-head equipment, and processing circuitry has enabled newer drives to accurately read data at much higher linear velocities then the original drives. These CDROM drives are classified by their maximum data transfer rate capability relative to the original drives, e.g., 2×, 4×, 8×, 12× (representing 12 times the original data transfer rate of 153,600 bytes per second), 24×, etc.
Another performance consideration is access time, also commonly referred to as seek time. Seek time is the time it takes to move from one data location on a disc to another data location on the disc. Listed seek times are generally a weighted average of seek times, and they give some indication of what the average seek time will be under typical use. Because the optical read-head of a CD drive is substantially more massive than the flyweight mechanism of hard discs, it takes significantly more time to precisely move the read-head to a new position. Furthermore, if the radial position (on the CD) of the new data to be read is a fair distance away from the present position of the read-head, an additional wait will be required to adjust the speed of the CD drive motor. Since the linear velocity under the read-head must be maintained at a constant rate in a CLV drive, any change in the radial position of the read-head requires a change in the speed of the motor. Since the inertia of the motor, platter, and CD is relatively large in comparison to the motor torque, it takes some time to change the speed of the motor, especially when moving from the inside of the CD to the outside or vice-versa.
CD-ROM drive manufacturers are constantly striving to reduce their manufacturing costs, increase drive performance, and increase drive reliability. While CLV drives work well for audio playback, their design is not optimized for minimum manufacturing costs, maximum reliability, and optimal data transfer rates. Because the speed of the motor must be constantly varied to obtain a constant read-head linear velocity, it is necessary to have more expensive motors and control circuitry than would be required if the motor could be spun at a single constant speed. This additional complexity and the constantly changing motor speed also leads to decreased drive reliability. In addition, the required change in motor speed leads to increased seek times.
In order to overcome these drawbacks, a new type of CD-ROM drive has recently been introduced. The advancements in technology discussed above have facilitated the development of CD-ROM drives that can read data at variable read-head linear velocities. These new drives are called Constant Angular Velocity or CAV CD-ROM drives. In a CAV CD-ROM drive the motor spins the disc platter at a constant angular velocity (i.e., a constant R.P.M). Since the CD is spun at a constant velocity, the linear velocity at which data passes under the read-head is proportional to the radial position of the read-head relative to the center of the disc. This means that for any given RPM, these new drives will have higher linear read-head velocities and therefore higher data transfer rates for read-head positions which are closer to the outside of the CD-ROM. CAV drives also have lower seek times than CLV drives during large read-head position changes since the drive motor speed does not have to change. Current generation CAV CD-ROM drives have seek times of about 100 ms. This is a significant improvement over older CLV drives.
It is important to note that the data transfer rate of the new CAV drives does not necessarily increase linearly as the drive-head moves to the outside of the disc. If the hole in the center of the CD-ROM is not well centered relative to the pit spiral on the disc, then the read-head tracking circuitry in some drives is not able to properly track the spiral as the read-head moves towards the outside of the disc. To compensate for this, some drives will switch back to CLV mode, or will reduce the speed at which they are spinning the motor in order to be able to better read the data on the outside of the disc. In addition, some CD-ROM discs are not well balanced and some drives are not capable of spinning them at the highest speed possible because the forces generated by an unbalanced disc increase with the speed at which a disc is spun.